At use of a pump in a reservoir, pump stations and such spaces which when necessary require discharge of liquid, such as waste water, penetrating ground water, etc., an as optimal operation of the pump as possible is aimed at, which inter alia imply that a sufficient amount of liquid is allowed to accumulate before the pumping/bilge pumping takes place in order to get the number of start and stop of the pump to a minimum.
Starting and stopping, respectively, of the pump takes place in an conventional way by determining the liquid level in the reservoir by means of a level sensor. These level sensors within the area of waste water treatment are usually so called free hanging mechanical level sensors, which take a small inclination in relation to a plumbline when the level sensor is surrounded by air and takes a large inclination in relation to the plumbline when the level sensor is immersed in the liquid. The orientation of the level sensor change from the first to the second position as the liquid level rise and from the second to the first position as the liquid level sink. At the time of such a change a movable weight enclosed inside the level sensor activates a micro switch whereupon the pump is started and stopped, respectively. One example on such a level sensor is shown in EP 0,568,508 B1.
This type of level sensor comprising movable weights are difficult to construct in order to obtain a reliable operation of the level sensor. Problems that may arise are that the movable weight can get stuck in different positions or that deposits from the liquid attach to the housing of the level sensor whereupon the inclination of the level sensor in relation to the plumbline decreases when it is immersed in the liquid or the inclination of the level sensor is obstructed by other components in the pump station, which result in an erroneous operation of the level sensor.
Thereto there are floating level sensor, which hangs more or less vertical when they are surrounded by air and which floats more or less horizontal when the liquid level is high enough. Due to currents in the liquid such level sensors may float about and get tangled in other level sensors, pipes, ladders, etc., which result in an erroneous operation of the level sensor.